debugcommands: add support for extensions adding their own debug info We've had a couple of cases where it'd be handy at Google to add data to `hg debuginstall`'s output. We've kludged around that at various times, but it seems reasonable to let extensions add their own data here so extension maintainers can get useful extra data. Differential Revision: https://phab.mercurial-scm.org/D6681